1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier and a printer and a process cartridge detachably attachable thereto, and more particularly, to an apparatus for recovering residual toner on an image bearing member by developing means after transfer.
2. Related Background Art
Recently, miniaturization of an image forming apparatus, such as an electrophotographic copier and a printer, has been proceeding. There have been limits to the miniaturization effected merely by miniaturizing each means in a process of forming an image such as electrifying, exposing, developing, transferring, fixing and cleaning.
In an image forming apparatus of the transferring type, some transfer residual toner on a photosensitive member (hereinafter referred to a photosensitive drum) as an image bearing member after transfer is collected by a cleaner as waste toner, which is preferably not produced due to its environmental impact.
This has caused the development of an image forming apparatus of a cleaner-less system, where a cleaner is removed to carry out cleaning simultaneous with developing of the transfer residual toner by a developing apparatus (developing device). The cleaning simultaneous with developing is a method of recovering some transfer residual toner on the photosensitive drum after the transfer by a fog clearing bias (a potential difference of the fog clearing bias xe2x80x9cV back,xe2x80x9d which is the potential difference between a direct voltage applied on developing means and a surface potential of the photosensitive drum) at the time of developing in and after a next process.
According to this method, the transfer residual toner is recovered by the developing means to be used in and after the next process, which produces no waste toner and reduces trouble in maintenance. Such a cleaner-less system has a great space advantage to permit the considerable miniaturization of the image forming apparatus. Further, an electrifying apparatus (electrifying device) of the contact-electrifying type permits recovering of the transfer residual toner by an electrifying member, which makes contact with the photosensitive drum and discharging the same again onto the photosensitive drum to be recovered by the developing means.
Moreover, the contact electrifying apparatus, namely an apparatus of a type that effects electrification by causing a voltage-applied electrifying member to abut against the photosensitive drum, has been in practical use as the electrifying means of the photosensitive drum because of its advantage of low power or the like.
As such a contact electrifying apparatus, a magnetic brush electrifying apparatus has been preferably used in terms of stability of an electrifying contact. In the magnetic brush electrifying apparatus, the electrification is started by magnetically attracting conductive magnetic particles, functioning as a magnetic brush, directly on a magnet or a sleeve containing the magnet, stopping or rotating the magnetic brush to be contacted with the photosensitive drum, and applying the voltage thereon.
Conductive fibers in the form of a brush (a fur brush) and a conductive rubber roller, which is a rolled conductive rubber have been preferably used as a contact electrifying member.
The electrifying apparatus of the magnetic brush type and, as the photosensitive drum, a normal organic photosensitive drum having a surface on which the conductive fine particles are dispersed or an amorphous silicon photosensitive drum, are used to make it possible to obtain, on the surface of the photosensitive drum, an electrifying potential substantially the same as a direct component of the bias applied on the magnetic brush as the contact electrifying member. Such an electrifying method is called injecting electrification. This injecting electrification utilizes, in the electrification to the photosensitive drum, no electric discharging phenomenon as in the case of using a corona electrifying device, these by permitting the electrification without ozone and by consuming a small amount of power, and attention has been paid thereto.
In the image forming apparatus using such an injection electrifying method, use of a spherical carrier granulated by polymerizing as the conductive magnetic particle (hereinafter referred to as an injection carrier) narrows an area in contact with the photosensitive drum and prevents reaching a desired potential to cause a defective image, such as fog or a ghost, when passing-sheet enduring is continued.
This brings the use of a carrier granulated by grinding (hereinafter referred to a ground carrier) instead of the spherical injection carrier. The ground carrier produces a finely powdered carrier, which enlarges the area in contact with the photosensitive drum and thereby increases an electrifying property of the photosensitive drum. Further, the ground carrier is easy to convey and produces large friction against the photosensitive drum, which facilitates recovering the transfer residual toner and increases the cleaning property.
When such injection electrification is used as the electrifying method, the injection carrier is preferably of grounded shape in order to prevent degradation of the image resulting from the poor electrification.
In the injection electrifying method, the photosensitive drum passes an electrifying portion (electrifying area or electrifying nip) which is a contacting portion with the magnetic brush as the contact electrifying member. If the electrification is carried out merely by applying a DC (direct current) bias, the potential difference between a carrying member for the magnetic brush (hereinafter referred to as an injection sleeve) and the photosensitive drum is wide when the photosensitive drum starts contact with the magnetic brush at the electrifying portion so that the resistance of the injection carrier is low to facilitate the flow of the current.
Just before the photosensitive drum is separated from the electrifying portion, however, the potential is on the surface of the photosensitive drum to decrease the potential difference between the injection sleeve and the photosensitive drum and to thereby increase the resistance of the injection carrier. It is known that this causes difficulty in the flow of the current and reduces the electrifying property of the photosensitive drum (FIG. 5).
For this reason, in the injection electrifying method, an AC (alternate bias) is often applied at the same time as applying the DC (direct bias applying) as the electrifying bias. Applying the AC decreases the resistance of the injection carrier to facilitate the flow of the current and to increase the electrifying property to the photosensitive drum, which permits the electrification to a desired potential.
However, the electrification by simultaneously applying the AC causes a micro potential difference between the photosensitive drum and the injection sleeve corresponding to a waveform of the AC. An example of this is shown in FIG. 6. For example, when the electrification is carried out at xe2x88x92500 V by applying the DC and simultaneously by applying the AC at 700 Vpp, there exist locally a potential differences of xe2x88x92150 V at the injection sleeve and xe2x88x92850V at the photosensitive drum. The potentials of the injection sleeve and the photosensitive drum are the same during the time when the photosensitive drum passes the electrifying portion, which is the contacting portion with the magnetic brush, and just after the photosensitive drum has passed the electrifying portion, phases of the potentials are shifted between the injection sleeve and the photosensitive drum. The shift of the phase occurs such that the injection sleeve at xe2x88x92150 V and the photosensitive drum at xe2x88x92850 V causes a potential difference at 700 V between the injection sleeve and the photosensitive drum. At this time, between the injection sleeve and the photosensitive drum, a voltage drop between the injection carrier and the photosensitive drum is the largest, so that the injection carrier is attracted to the photosensitive drum. This phenomenon is called xe2x80x9ccarrier adhesionxe2x80x9d.
When the injection electrification is used as the electrifying method, simultaneously applying the AC is preferable and some occurrence of such carrier adhesion cannot be avoided during the injection electrification.
When the injection carrier, after carrier adhesion, reaches a transferring portion, the injection carrier enters a nip between a transferring member and the photosensitive drum, which results in damaging the photosensitive drum.
The potential has difficulty reaching a once damaged portion of the photosensitive drum so that the potential difference between the injection sleeve and the photosensitive drum is increased to acceleratedly increase the carrier adhesion. Repeating a passing-sheet test then further deepens the damage of the photosensitive drum and the potential does not reach the damaged portion so that the phenomenon of streak-like developing (hereinafter referred to xe2x80x9cstreak-like fogxe2x80x9d) occurs.
When the injection carrier is changed in its shape from spherical to ground in order to increase the electrifying property, the particle of the ground carrier has a large number of edges in contact with the photosensitive drum at points, so that pressures thereon are also increased. Accordingly, the use of the ground carrier especially often damages the photosensitive drum to easily cause a defective image by the damage before twenty thousand sheets are passed in the passing-sheet test.
In the light of the above condition, Japanese Patent Application Laid-Open No. 59-133569 discloses a mechanism of trapping magnetic fine particles that abuts a blade against the photosensitive drum on an downstream side in a rotational direction of the photosensitive drum with respect to a magnetic brush electrifying member to trap and recover the magnetic fine particles separated from a magnetic brush and moved to the photosensitive drum. In this case, when the ground carrier enters a nip between the blade and the photosensitive drum, the carrier is likely to damage the surface of the photosensitive drum, and its influence is much stronger especially in the photosensitive drum, which surface is not very hard. At this time, the use of such a blade damages the photosensitive drum and causes the production of a defective image when the passing-sheet test of ten thousand sheets is performed.
In the cleaner-less system that recovers the transfer residual toner by the magnetic brush electrifying member and discharges the same again onto the photosensitive drum to be recovered again by the developing apparatus, the toner discharged from the magnetic brush is also damaged by the above described blade to cause remarkable contamination from the toner in the electrifying apparatus. This prevents the electrification to the desired potential, resulting in a defective image, such as fog or a ghost, due to the poor electrification when passing-sheet test of ten thousand sheets is performed.
In the above-described electrifying apparatus, the transfer residual toner is once recovered in the contact electrifying member and then discharged onto the image bearing member by a potential operation to be recovered by the developing apparatus. However, all the transfer residual toner having reached the above-described electrifying apparatus is not ideally discharged, with the normal charge maintained, onto the image bearing member and passed on the image bearing member to be recovered by the developing apparatus. The remaining toner is finally accumulated in the electrifying apparatus or spattered from the contact electrifying member or the image bearing member to contaminate the inside of a body of the image forming apparatus. Especially in a system where exposing means, such as an LED, is arranged near the electrifying apparatus, the spattered toner attached to a surface of the LED to thereby shut off exposure, will cause an undesirable lack of an image.
Further, the toner, to which the normal charge is not applied, among the toner discharged onto the image bearing member, is neither recovered in the developing apparatus in a developing area nor transferred to move around on the image bearing member, thereby causing disadvantages such as spattering or shutting off the exposure when its concentration is increased.
An object of the present invention is to provide an image forming apparatus and a process cartridge detachably attachable thereto that prevents damage to an image bearing member by a conductive particle and contamination from toner by electrifying means.
Another object of the present invention is to provide an image forming apparatus and a process cartridge detachably attachable thereto which prevents spattering of the toner from the electrifying means.
Still another object of the present invention is to provide an image forming apparatus comprising: an image bearing member which is movable; electrifying means which makes contact with the image bearing member and electrifies the image bearing member; developing means for developing a latent image on the image bearing member by toner; and an abutting member which is provided on a downstream side in a moving direction of the image bearing member with respect to the electrifying means and a contacting portion of the image bearing member and abuts against the image bearing member, the image bearing member electrified by the electrifying means being exposed to have the latent image formed thereon, a toner image formed on the image bearing member by the developing means being transferred to a recording material, and the developing means recovering residual toner on the image bearing member after transfer.
A further object of the present invention is to provide an image forming apparatus and a process cartridge detachably attachable thereto comprising: an image bearing member which is movable; electrifying means which has a carrier for carrying a conductive particle and interposes the conductive particle between the carrier and the image bearing member to electrify the image bearing member; developing means for developing a latent image on the image bearing member by toner; and an abutting member which is provided on a downstream side in a moving direction of the image bearing member with respect to the electrifying means and a contacting portion of the image bearing member and abuts against the image bearing member, the conductive particle of the electrifying means making contact with the image bearing member, the image bearing member electrified by the electrifying means being exposed to have the latent image formed thereon, a toner image formed on the image bearing member by the developing means being transferred to a recording material, the developing means recovering residual toner on the image bearing member after transfer, and the pressure of the abutting member against the image bearing member being 0.3 to 3 kPa.
A still further object of the present invention is to provide an image forming apparatus and a process cartridge detachably attachable thereto comprising: an image bearing member which is movable; electrifying means which makes contact with the image bearing member and electrifies the image bearing member; developing means for developing a latent image on the image bearing member by toner; and an abutting member which is provided on a downstream side in a moving direction of the image bearing member with respect to the electrifying means and a contacting portion of the image bearing member and abuts against the image bearing member, the image bearing member electrified by the electrifying means being exposed to have the latent image formed thereon, a toner image formed on the image bearing member by the developing means being transferred to a recording material, the developing means recovering residual toner on the image bearing member after transfer, and the abutting member being provided on the electrifying means.
A yet further object of the present invention is to provide an image forming apparatus and a process cartridge detachably attachable thereto comprising: an image bearing member which is movable; electrifying means which has a carrier for carrying a conductive particle and interposes the conductive particle between the carrier and the image bearing member to electrify the image bearing member; and developing means for developing a latent image on the image bearing member by toner, the image bearing member electrified by the electrifying means being exposed to have the latent image formed thereon, a toner image formed on the image bearing member by the developing means being transferred to a recording material, the developing means recovering residual toner on the image bearing member after transfer, and the electrifying means having a regulating member for regulating the amount of the conductive particle on the carrier and a shutting-off member for shutting off spattered toner provided on a downstream side in a moving direction of the carrier with respect to the regulating member.